In an engine system, an electronic control unit (ECU) of a gasoline engine outputs an ignition signal, and an igniter (electronic ignition circuit) of the engine causes an ignition plug of the engine to discharge by means of an ignition coil of the engine based on the ignition signal. The igniter generates a fail-safe signal indicating whether the ignition plug has actually discharged by means of the ignition coil. The igniter feeds back the fail-safe signal to the ECU. This is disclosed in JP 8-93611A.
One example of a circuit for outputting a fail-safe signal is shown in FIG. 5. A transistor Tr1 outputs a fail-safe signal IGf through its collector to a signal wire LF. The emitter of the transistor Tr1 is grounded. A pull-down resistor Rpd is connected between the base and emitter of the transistor Tr1. The base of the transistor Tr1 is connected to a power source B via the emitter and the collector of a transistor Tr2. The base of the transistor Tr2 is grounded via the collector and the emitter of a transistor Tr3.
It is possible to switch the output of the transistor Tr1 by turning on or off the transistor Tr3. That is, it is possible to determine the logical value of the fail-safe signal IGf by turning on or off the transistor Tr3. When the transistor Tr3 is turned off, the transistor Tr2 is turned on, so that a current flows through the pull-down resistor Rpd. As a result, a bias in proportion to the voltage drop across the pull-down resistor Rpd is applied between the base and the emitter of the transistor Tr1. This turns on the transistor Tr1, making the fail-safe signal IGf logically low. When the transistor Tr3 is turned on, the transistor Tr2 is turned off, so that the pull-down resistor Rpd short-circuits the base and emitter of the transistor Tr1. This turns off the transistor Tr1, making the fail-safe signal IGf logically high.
However, when a radio noise is superposed on the signal wire LF when the transistor Tr3 is in the OFF-state, a current may flow through the pull-down resistor Rpd via the parasitic capacitance between the collector and the base of the transistor Tr1. In this case, a bias in proportion to the voltage drop across the pull-down resistor Rpd is applied between the base and the emitter of the transistor Tr1, so that this transistor Tr1 may malfunction. When the resistance of the pull-down resistor Rpd is reduced, the malfunction of the transistor Tr1 due to radio noises could be suppressed. In this case, however, the current required for the pull-down resistor Rpd to turn on the transistor Tr1 increases. When a high-capacitance capacitor is connected to the collector of the transistor Tr1, the malfunction of this transistor due to radio noises could be avoided, but the igniter would be large in size.
In general, although the output circuits of not only the igniter but also on-vehicle electronic devices are downsized, it is difficult to avoid the malfunction of the circuits due to radio noises.